THE ORGANIC MATTER OF THE SOIL 123 



organic matter may perform its legitimate functions. The 

 source of supply will be considered first. 



The organic matter of the soil may be increased in a nat- 

 ural way by the plowing under of green crops. This is 

 called green-manuring and is a very satisfactory practice. 

 Such crops as rye, buckwheat, clover, peas, beans, and vetch 

 lend themselves to this method of soil improvement. Not 

 only do these crops increase the actual organic content of 

 a soil, but in the case of legumes the nitrogen may also be in- 

 creased in amount, if the nodule bacteria are present and 

 active. 



Green-manures to be effective must be hardy, rapid in 

 growth, succulent, and should produce abundant foliage. Rye 

 and oats are particularly valuable from this standpoint. Such 

 legumes as cowpeas, vetch, field peas, soybeans, and velvet 

 beans are adapted to summer growth. Red clover or sweet 

 clover, being a biennial, may be seeded one year and turned 

 under the next spring. Oats and peas or rye and peas make 

 a very good combination for fall green-manuring. Hairy or 

 winter vetch may be seeded with rye in the autumn and used 

 as a green-manure in the spring. In the South green-manur- 

 ing crops may be utilized to much better advantage than m the 

 northern states as the longer growing season permits the use 

 of a green-manure following the normal harvest. 



Due to the tendency of bare soil to lose nutrients by leach- 

 ing, especially in the summer and fall, it is always best to 

 keep the land covered with vegetation of some kind. Cover- or 

 catch-crops are used for this purpose, especially on sandy land, 

 although they are profitable on heavier soils as well. Wheat 

 on sandy land may be followed by cowpeas, which not only 

 conserve nitrates but fix nitrogen from the air in addition. 

 Rape, cowpeas, vetch, and soybeans are sometimes seeded in 

 corn at the last cultivation. "When a soil receives clean culti- 

 vation a part of the year, as is practiced very frequently in 

 orchards, it is very desirable that a crop be plowed under oc- 



